In the past it has been common practice to measure through analog methods the impedance or current across a given telephone line in an attempt to determine when a telephone device has bridged or has been connected to the line. This method was also attempted in trying to determine if an additional telephone device has answered the line or gone off-hook while at the same time another device is already using the telephone line. This condition is attributable to such situations as a wire tap condition when other listening or answering devices are present on the same telephone line or when an extension telephone attempts to access a telephone line when the line is already in use.
Other existing telephone devices use a similar type of detection circuit to determine if the called telephone line is answered by a telephone set or in the absence of detecting the off-hook condition would then determine that an answering machine or similar device should respond and connect the line. In this way, answering machines and similar devices are capable of sequencing a response so as to provide a desired answering or connecting function.
During the course of recent years, it has become also quite popular to use microprocessors in the type of machines which answer and respond to telephone lines. In some cases, the microprocessor is arranged to directly measure the actual voltage that is on the telephone line when the ring voltage is present. When a telephone set or other telephone responding device answers the line the ring signal is interrupted indicating that the line has been answered. Various other devices have been suggested to sense or measure a change in the line voltage which is present on the telephone line in order to determine if and when the line has been answered.
It is now becoming evident that a more sensitive and reliable, and yet inexpensive, circuit is required in order to be able to detect when a telephone device has gone off-hook especially when the line has already been answered by another device. This is true when it is necessary to control or sequence the presence of various devices on the same telephone line. Through the use of a bridging sensor circuit, it is feasible that sequential control of the telephone line can be accomplished to allow proper operation so that the same telephone line can be used for multipurposes. In this way, a single telephone line can be used for the sequenced transmission of voice or data through either a telephone set, facsimile machine, computer modem, point of sale register or other similar device. This arrangement can save considerable expense in eliminating the requirement for dedicated telephone lines if a single telephone line can be used for all of these functions.
In most prior art arrangements, it is necessary to connect a branching switch directly to the telephone line and then, selectively switch the various devices as required. This arrangement necessitates the rewiring of the telephone system for the connection and positioning of the branching switch. It is highly desirable to allow the branching switch to control the individual telephone line, but also eliminate the necessity for rewiring and installing the switch on an existing telephone line ahead of all existing down-line devices.
It is an object of the present invention to accurately and quickly sense when a telephone device and any number of other telephone devices connected to the same telephone line go off-hook simultaneously or at various times. By sensing this condition, it is possible for a branching switch to sequentially control the connection of the various telephone devices so that one will not interfere with the other so as to obtain maximum use of the connected telephone line.
It is a further object of the present invention to provide a bridging sensor which is more accurate and fool-proof than existing analog or digital sensors.
It is a still further object of the present invention to provide an off-hook sensor with improved sensitivity and accuracy and which is not subject to long term drift as a result of sensor or component tolerances or environment changes.
It is a still further object of the present invention to provide a sensor which detects when a bridging telephone device goes back to the on-hook condition.